A recently developed noncontact tracing control system utilizing a noncontact distance detector for tracing the contour of a model uses an optical distance detector fixed at the tip end of a tracer head and detecting a distance to the model surface, for a tracing thereof. This system eliminates worry about damage to the model, and therefore, a soft material model can be used, and thus the applicability thereof in tracing machining is expected to expand.
Nevertheless, the conventional noncontact tracing control system has a problem in that the tracing accuracy is low at portions of a model where an inclination thereof is large. Namely, in such a portion, an optical measuring axis of a distance detector becomes almost parallel to the model surface, whereby a spot on the model surface is expanded and becomes an ellipsoid, and thus the resolution of the distance detector is lowered, and accordingly, the tracing accuracy also is lowered. Particularly for a trigonometrical distance detector, a measurement sometimes becomes impossible because, depending on this angle, the optical measuring axis interferes with the model surface.
To solve this problem, two detectors are used, the measurement is conducted a number of times, three optional points are selected from among the plurality of points obtained, a normal vector of a model surface is acquired from these three points, and the distance detectors are rotated in the direction of this normal vector, to thereby improve the measurement accuracy. An example of the above is disclosed in Japanese Patent Application No. Hei 1-194500.
Nevertheless, when three points are selected at random from among the plurality of points obtained, such points might be on a straight line or in the neighborhood of a straight line, and an accurate normal vector on the model surface might not be acquired; i.e., if the three points are on a straight line, a normal vector cannot be obtained. Moreover, if they are in the neighborhood of a straight line, at the portion where the rate of change of the curvature of an actual model surface is large, the surface obtained is entirely different from an approximate plane of the model surface.